Uranium dioxide article and method of producing the same

ABSTRACT

The invention relates to the production of uranium dioxide articles such as nuclear fuel elements and ceramic shapes and which comprise a powdered mixture of UO2 X and U or Pu metal, wherein X is the amount of excess O2 required to produce, from said mixture, stoichiometric UO2 when subjected to reaction conditions under irradiation. One of the problems incidental to the use of nuclear fuel elements containing vibration packed mixtures of uranium dioxide (UO2) powders is the shrinkage of the UO2 that occurs at the high temperatures generated in the fuel element during irradiation. The maximum packed densities thus far obtained in nuclear fuel elements, made by vibration packing inside a metal sheath a mixture of different size fractions of UO2 powders, are in the range of 88-92 percent of the theoretical density. The UO2 powder is commonly prepared by crushing previously sintered or fused blocks of UO2. During irradiation of such an element, the UO2 powder tends to sinter with resultant shrinkage of the UO2 body . This shrinkage could impair heat transfer between fuel and sheath and might eventually result in the collapse of the fuel-sheathing when this is insufficiently thick to be self-supporting in the reactor coolant pressure.

United States Patent 72] lnventor Brian S. Wyatt Deep River, Ontario, Canada [21] Appl. No. 801,809 [22] Filed Feb. 24, I969 [45] Patented {73] Assignee Sept. 28, 1971 Atomic Energy of Canada Limited Ottawa, Ontario, Canada [54] URANIUM DIOXIDE ARTICLE AND METHOD OF PRODUCING THE SAME 7 Claims, No Drawings '7 [52] US. Cl 25Z/30I.l, 23/344, 23/355, 264/05 [51] Int. Cl C09k 3/00 [50] Field ofSearch 252/301.1; 23/344, 346, 355; 264/05 Mateke, Hs., Nuc. Sci. Abstracts, Vol. 23, No. 17, 34101, p. 3514.

Primary Examiner-Carl D. Quarforth Assistant Examiner-R. L. Tate Attorney-Weir, Marshall, MacRae & Lamb ABSTRACT: The invention relates to the production of uranium dioxide articles such as nuclear fuel elements and ceramic shapes and which comprise a powdered mixture of U0, and U or Pu metal, wherein X is the amount of excess 0 required to produce, from said mixture, stoichiometric U0 when subjected to reaction conditions under irradiation.

One of the.problems incidental to the use of nuclear fuel elements containing vibration packed mixtures of uranium dioxide (U0 powders is the shrinkage of the U0, that occurs at the high temperatures generated in the fuel element during irradiation. The maximum packed densities thus far obtained in nuclear fuel elements, made by vibration packing inside a metal sheath a mixture of different size fractions of U0 powders, are in the range of 88-92 percent of the theoretical density. The U0 powder is commonly prepared by crushing previously sintered or fused blocks of U0,. During irradiation of such an element, the U0, powder tends to sinter with resultant shrinkage of the U0, body This shrinkage could impair heat transfer between fuel and sheath and might eventually result in the collapse of the fuel-sheathing when this is insufficiently thick to be self-supporting in the reactor coolant pressure.

URANIUM DIOXIDE ARTICLE AND METHOD OF PRODUCING THE SAME The present invention seeks to provide a uranium dioxide article containing a body of U which is not subject to shrinkage under sintering conditions.

The invention may be said to reside in the production of a ceramic shape of fuel element comprising a mixture of hyperstoichiometric uranium dioxide (UO and uranium (U), or plutonium (Pu) metal powders wherein X is the amount of excess 0 required to produce, from the mixture, stoichiometric of nearstoichiometric U0 The invention may also be said to reside in a method of producing a uranium dioxide article which comprises providing U e.g., by oxidizing U0 to produce U0 and preparing a mixture of powders of said U0 and U or Pu metal wherein the amounts of excess 0 represented by X and said metal are such as to produce from said mixture stoichiometric U0, on subjection to reaction temperatures, i.e., temperatures within the range 600l200 C.

The invention is based upon the possibility of equilibrating mixtures of UO and uranium or plutonium metal to obtain stoichiometric or near stoichiometric U0 This reaction would result in an increase in the actual solid volume of the mixture and counteract shrinkage.

in accordance with the invention, therefore, a mixture of U0, and uranium or plutonium metal powders is formed, such mixture having incorporated therein sufficient excess oxygen to produce a stoichiometric or nearstoichiometric mixture. While theoretically the excess oxygen could be added by either oxidizing the U0 or adding oxygen to the mixture by some other means, this invention proposes to incorporate the oxygen in combined form in the matrix by oxidizing the Uo since free oxygen would tend to diffuse more readily into the sheathing material in which the mixture may be vibration packed. The O/U ratio in the U0 would ordinarily be in the range of about 2.01 to 2.30. The uranium metal in the mixture comprises about 31 2 percent thereof.

Assuming, for instance, that the U0 has been oxidized to a composition of U0 the proportion of uranium metal (U) to uranium oxide (U0 in the mixture should be such that:

tw U suo, Weight g. 2 l92 235 2430 Vol. cm. 196.5 12.5 222 As indicated, an initial volume of 209 cm. of U+UO would swell to 222 cm. on equilibration, when the U content of the initial mixture was 9.8 wt. percent. The theoretical density of the UO,/U mixture would be about percent higher than that of stoichiometric U0 Thus, it is possible, using the same vibratory compaction process, to pack-the powder mixture to about 5-10 percent higher density than stoichiometric U0 and, with some restraint, prevent swelling on equilibration.

The mixture of U0 and U metal powders is prepared in conventional particle-size ranges and, as nuclear fuel ele ments, is vibration packed in metal sheaths to a density of about 9.710.96 g./cc. in the usual manner to form the fuel elements.

When fused U0, is used, it is crushed to an appropriate range of sieve sizes to achieve efiective vibration packing.

Experiments to equilibrate U and U0 at 800 C. and 1,000 C. to make stoichiometric mixtures have shown that the initial U is converted to U0 and increases in volume, while becoming metallurgically bonded to the initial UO substrate. These experiments have also shown that because of the increase in volume accompanying the reaction, there is no tendency for the U0 product to shrink away from the sheathing wall in fuel elements.

The invention may also be applied to the production of large complex ceramic shapes of U0; where shrinkage during sintering would produce unacceptable distortion in the finished article. ln this case, the reaction would be utilized to counteract shrinkage during sintering. A mixture of the UIUO powder can be vibration packed in a container of, for instance, M0 or Mo-lined graphite, then annealed in an inert atmosphere at a suitable temperature to equilibrate the U and U0 In this manner, coherent compacts may be produced to required dimensions with avoidance of the problem of shrinkage and attendant dimensional control in their fabrication.

lclaim:

1. An article comprising a compacted mixture of powders of uranium dioxide containing excess oxygen (UO and uranium (U) or plutonium (Pu) metal, wherein X is the amount of excess oxygen required to produce from said mixture, when the latter is subjected to an elevated temperature at which reaction occurs, stoichiometric uranium dioxide and, when plutonium is present in said compacted mixture, stoichiometric plutonium dioxide.

2. An article as claimed in claim 1, including a sheath wherein said mixture is vibration packed to a density in the range of9.70 to 10.96 g./cc.

3. An article as claimed in claim I, wherein the OlU ratio in said UO is in the range of 2.01 to 2.30.

4. An article as claimed in claim 1, wherein said U or Pu metal constitutes from 3 to 12 percent by weight of said compacted mixture.

5. A method of producing an article which comprises compacting a mixture of powders of U0 and U or Pu metal, wherein X is the amount of excess oxygen required to produce from said mixture, when the latter is subjected to an elevated temperature at which reaction occurs, stoichiometric uranium dioxide and, when plutonium is present in said compacted mixture, stoichiometric plutonium dioxide.

6. A method according to claim 5, wherein said mixture is compacted into a container.

7. A method according to claim 5, including the step of vibration packing said mixture in a sheath to a density in the range from 9.70 to 10.96 g./cc. 

2. An article as claimed in claim 1, including a sheath wherein said mixture is vibration packed to a density in the range of 9.70 to 10.96 g./cc.
 3. An article as claimed in claim 1, wherein the O/U ratio in said UO2 X is in the range of 2.01 to 2.30.
 4. An article as claimed in claim 1, wherein said U or Pu metal constitutes from 3 to 12 percent by weight of said compacted mixture.
 5. A method of producing an article which comprises compacting a mixture of powders of UO2 X and U or Pu metal, wherein X is the amount of excess oxygen required to produce from said mixture, when the latter is subjected to an elevated temperature at which reaction occurs, stoichiometric uranium dioxide and, when plutonium is present in said compacted mixture, stoichiometric plutonium dioxide.
 6. A method according to claim 5, wherein said mixture is compacted into a container.
 7. A method according to claim 5, including the step of vibration packing said mixture in a sheath to a density in the range from 9.70 to 10.96 g./cc. 